This section provides background information related to the present disclosure which is not necessarily prior art.
FIG. 1 shows an example of a prior art single module uninterruptible power supply (“UPS”) system 100, which is referred to herein as a UPS module 100. The basic elements of this UPS module are a rectifier 102, an inverter 104, a back-up DC power source 106 (a battery in this case), a control unit 108 and a bypass switch 110. An input 112 of rectifier 102 is coupled to a rectifier power source 114 and an output 116 of rectifier 102 is coupled to a DC bus 118. DC bus 118 is coupled to an input 120 of inverter 104 and to back-up DC power source 106. An output 122 of inverter 104 is coupled to a load (or loads) 124 that are powered by UPS module 100. An input 126 of bypass switch 110 is coupled to a bypass power source 128 and an output 130 of bypass switch 110 is coupled to the output 122 of inverter 104. UPS module 100 may also include an output transformer (not shown) coupled between the output 122 of inverter 104 and load 124. Under normal operating conditions the output voltage of the inverter 104 is synchronized by the control unit 108 to the voltage of bypass power source 128, as shown in FIG. 2. That is, the output voltage of inverter 104 is controlled so that it is in phase with the voltage of bypass power source 128. When the bypass power source 128 is not qualified, for example goes to zero volts, the inverter 104 will output a voltage that is at a nominal frequency of bypass power source 128 but not locked in phase to any specific source. It should be understood that the term “qualified” as used with reference to a bypass power source 128 has its conventional meaning in the context of uninterruptible power supply systems. That is, a bypass power source is qualified when the power it is providing is within acceptable limits of its nominal operating parameters, such as voltage, frequency and phase rotation.
In an effort to increase availability, a plurality of UPS modules 100 can be connected to a device called a static transfer switch 300 as shown in FIG. 3 controlled by a control unit 302. Static transfer switch 300 switches a load 308 coupled to an output of static transfer switch 300 among one or more of the UPS modules 100 coupled to static transfer switch 300. This arrangement is referred to herein as multiple UPS system 304. In this arrangement, one of UPS modules 100 (referred to herein as UPS 1A) would be the sync master and its control unit 108 would send out a synchronization signal on a synchronization line 305 of a synchronization bus 306 to the control unit 108 of a slave UPS module 100 (referred to herein as UPS 1B). The output of inverter 104 of UPS 1B is be controlled by the control unit 108 of UPS 1B to be synchronized with the output of UPS 1A. That is, control unit 108 of UPS 1B controls the inverter 104 of UPS 1B so that the output voltage of inverter 104 of UPS 1B is in phase with the output voltage of UPS 1A. The bypass power source 128 for UPS 1A is referred to herein as bypass power source 128A and the bypass power source 128 for UPS 1B is referred to herein as bypass power source 128B.
If the synch master UPS module 100 (UPS 1A) loses its bypass power source 128A, it would go into a free run mode, but the slave UPS module 100 (UPS 1B) would still follow the output of UPS 1A. The result would be that UPS 1B would not be in sync with its bypass power source 128B and hence could not transfer to the bypass power source 128B if need be. FIG. 4 shows UPS 1B being out of phase with respect to the bypass power source 128B for UPS 1B.